This invention deals with long range laser ranging, and more specifically, with the accurate measurement of the separation between two points very far removed one from the other. To this end, it is known to place a transmitter-receiver laser unit at one of these points and a retroreflector target at the other point. The distance measurement is derived from the time required for a laser pulse to travel in one direction and then in the reverse direction, between the transmitter-receiver and the target.
As an example, one may have to measure distances of several hundreds (or even thousands) of kilometers with an accuracy on the order of a few centimeters. Typically, the distance between a point on the Earth and an object in space (a space vehicle or a natural satellite (Moon) or an artificial satellite orbiting the Earth), or generally, between two space-borne bodies, such as a space vehicle orbiting another planet, and the like, may need to be measured. By accumulating such measurements and using several targets, for example, the distances between several points on the terrestrial globe may then be determined with high accuracy using triangulation or similar techniques.
In practice, the retroreflector target embodies one or several retroreflectors, which are preferably composed of three mutually contiguous and orthogonal plane reflector faces, and the diagonal of the imaginary cube to which this cube corner belongs is a reference axis referred to as the retroreflector normal. A property of such a retroreflector target is to reflect incident rays back parallel to themselves. Thus, reflecting the laser beam back to the transmitter, even over great distances, does not require pointing the cube corner axis towards the transmitter, provided that the laser pulse has penetrated the cube corner, and of course, that the orthogonality of the reflecting surfaces is as perfect as possible.
In fact, the pulse reflected by a perfect cube corner has only one diffraction lobe, with an energy peak in the reflection direction whose equivalent beam width can be given, as a first approximation, by the relation .lambda./d, where .lambda. is the wavelength of the incident pulse, and d is the average transverse dimension of the target (improperly called the diameter). Thus, for a 0.5 .mu.m wavelength (green in color) and a target diameter of 10 cm, the lobe width, (in the absence of any disturbing medium) is about one arcsecond. This width is actually the subtended angle through which an observer placed at the target location would see this reflected pulse.
As long as the subtended angle through which the target sees the separation between the transmitter (at the time when the incident pulse is transmitted) and the receiver (at the time when the reflected pulse reaches it) is smaller than the lobe width, the ranging principle indicated above may be advantageously employed.
However, the received energy to transmitted energy ratio of the transmitter/receiver unit decreases whenever there is a large relative velocity between this transmitter/receiver unit and the retroreflector target, transversal to the direction of a straight line which would join them. In this case, it is known to define a velocity aberration angle, which depends on the ratio between the transverse relative velocity and the laser beam velocity (or the velocity of light). When this velocity aberration angle becomes greater than the lobe width, this means that the receiver is transversally deviated from the diffraction lobe of the return pulse, when the latter reaches the location previously occupied by the transmitter/receiver unit at the instant of pulse emission.
To compensate for this velocity aberration, it has already been proposed to change by a few arcseconds the right angles between the three reflecting faces, so as to widen the return beam. However, in practice, when this modification angle is increased from zero, the diffraction lobe, which was originally unique in a three-dimensional graph correlating the energy density transmitted by the target in one direction with two tilt angles characterizing the spatial orientation of this direction relative to the incident pulse direction, widens by having in its center a null surrounded by a regular ring; specifically, this ring is formed of six peaks arranged into a circle and interpenetrating each other.
Within a given retroreflector, one thus obtains an "omnidirectional" correction of velocity aberration, which however becomes insufficient when the latter substantially exceeds the mean width of the six individual lobes, because any additional increase in these modification angles results in the breaking-up of the above-mentioned ring into six separate lobes. The compensation effect becomes uncertain according to whether or not the receiver intercepts one of the six lobes; moreover, when, by chance, the receiver does intercept one of the six lobes, the light energy is reduced as it amounts to only a sixth of the total energy.
It has thus been proposed to provide a target with a plurality of small retroreflectors oriented randomly about their normals so as to generate an overall return pulse formed of a plurality of unit pulses, the sets of six lobes of which would be mutually complementary and would form together a ring-like lobe. However, the provision of several retroreflectors contributing to the formation of this overall lobe adversely affects the ranging accuracy, by virtue of the differences in the position of those retroreflectors over the target which, in particular, induce a time-wise spreading of the pulse arrivals at the receiver, and of the small size of these retroreflectors, which limits the individual energy of these return pulses.
The purpose of this invention is to mitigate these drawbacks and, even for large transverse velocities, to ensure an efficient correction of velocity aberration and to obtain high accuracy measurements, while maintaining a high received light energy to transmitted energy ratio.
In order to do so, the invention abandons the symmetry principle satisfied up to now in the field of laser ranging, where the orientation of cube corners about their normals did not matter, and where the same requirements applied to the three dihedral angles.